The present invention relates to polyisocyanate compositions containing toluene diisocyanate and mixtures of diphenylmethane diisocyanate isomers and polyphenyl polymethylene polyisocyanates that provide improvements in the tensile properties of flexible polyurethane foams.
German Offenlegungsschrift 2,221,811 describes the production of a molded flexible polyurethane foam using a mixture of 60 parts by weight toluene diisocyanate ("TDI") and 40 parts by weight of a mixture of diphenylmethane diisocyanates and polyphenyl polymethylene polyisocyanates ("crude MDI") (NCO content 31%). The molded flexible foam obtained in this way is particularly distinguished by favorable flameproof properties, although mechanical properties are not up to today's standards.
In addition, German Patentschrift 1,243,865 describes the production of polyurethane foams using TDI/MDI mixtures having MDI concentrations of 10 to 90% by weight. The crude MDI used contains 40 to 60% by weight of monomeric isomers having low concentrations of the 2,4'-isomer. Although these polyisocyanates are widely used in the automotive and furniture industries, the elongation at break of the polyurethane foams produced from these polyisocyanates is often unsatisfactory, with the result that standard automobile specifications cannot be satisfied.
It has now surprisingly been found that the use of special TDI/MDI mixtures can provide distinct improvements in the tensile properties of flexible polyurethane foams. These new TDI/MDI mixtures differ from the previously known mixtures in their relatively high content of monomeric MDI isomers, more specially 2,4'-MDI.